Multi-frequency induction heating and melting is known in the art. See for example U.S. Pat. No. 2,444,259, which is titled Method of High Frequency Induction Heating. Different frequencies result in different depths of induced eddy current heating in the workpiece, susceptor, or electrically conductive load placed in a crucible; the higher the frequency, the lower the effective induced eddy current depth of the current in the workpiece, susceptor or electrically conductive load. Multi-frequency currents may be applied (1) simultaneously or sequentially and (2) to a single or multiple induction coils that are disposed around the workpiece, susceptor or crucible in which the electrically conductive material is placed.
For an electrically conductive material, such as a metal composition that is placed in a crucible, the combination of low and high induction frequencies may be desirable to melt the metal at a high frequency and to stir the metal at a low frequency.
For the geometry of some workpieces, the combination of low and high induction heating frequencies is desirable. For example to metallurgically harden gear teeth it is known that a relatively low frequency (e.g., 3 kilohertz to 10 kilohertz) with relatively deep penetration of the induced eddy current into the gear is preferred to preheat the gear while a relatively high frequency (e.g., 30 kilohertz through 100 kilohertz) with relatively shallow penetration of the induced eddy current into the gear is preferred for final induction heating.
One objective of the present invention is to provide induction power at multiple frequencies to a workpiece with efficient transfer means between the multiple frequencies.